This invention relates in general to a technique for providing a unique identification code for audio material and more particularly for such audio material as is to be broadcast or otherwise transmitted over a medium which generates distortion and noise. For example, this invention permits identifying an advertising commercial that is to be broadcast on radio or television.
There are a number of applications that this invention may have. The one for which it has been specifically designed is for the identification of broadcast material over radio and television. Accordingly, the embodiment described herein will deal with that application of the invention.
A number of systems have been proposed or developed for transmitting auxiliary information along with the main program being broadcast. Super-audible and sub-audible sub-carrier transmission has been used in the prior art for achieving such multiplexing of an allocated broadcast channel. Some idea of the scope of techniques employed can be obtained from a review of U.S. Pat. Nos. 2,766,374, 3,061,783 and 3,391,340. These known techniques are not particularly well adapted for the transmission of unobtrusive signals for identifying or verifying the transmission or reproduction of particular programs. Applicant believes that the most pertinent known prior art system to applicant's invention is that disclosed in U.S. Pat. No. 3,845,391 issued on Oct. 29, 1974 to Murray G. Crosby. The Crosby system employs a short duration, narrow frequency notch in which a substantially inaudible code is located. The code is carried on a sub-carrier through a frequency shift keying (FSK) modulation technique. The sub-carrier is in the frequncy notch. The program is encoded at its beginning and its end. The decoder employed to detect and decode the encoded program employs an appropriate band pass filter that substantially matches the band stop filter employed in the encoder which generates the notch.
The embodiment of the Crosby System disclosed in the Crosby patent requires a code level that tends to be audible particularly where it must overcome the noise and distortion which occurs in connection with some audio broadcasting.
For example, audio broadcasters generally employ a technique of compressing the amplitude differences in the audio signal and transmitting the compressed audio information at the maximum allowable power level. This provides a better signal to noise ratio. It also provides a louder sound at the receiver. And, to some extent, it extends the range of the broadcast station without exceeding the power allocation. One result of this broacasting technique is that material is distorted. This distortion may not be perceived by the ear of a listener, but it is particularly severe on a low power level code residing in a narrow frequency notch.
Accordingly, it is the purpose of this invention to provide an improved encoding technique which will be effective at higher levels of program distortion and higher levels of background noise and non-code audio information than was hitherto the case.
It is a further purpose of this invention to provide these improved characteristics in a context that does not degrade the program material.
It is a further purpose of this invention to provide this enhanced coding technique in equipment that is sufficiently simple, reliable and inexpensive so that it can be used on a wide scale in a number of different types of media.
It is also a purpose of this invention to provide such a technique as will be adapted to automatic detection. Because of this automatic detection, it is important that the technique be relatively forgiving of a wide range of ambient conditions and transmission conditions and also require a decoder that is relatively trouble free.